Oscillator circuits are used in two main application categories. One of these applications is as a timekeeper or clock signal generator and the other is as a circuit for making signal frequency translation possible in telecommunication devices.
Electro-mechanical resonators, such as quartz resonators, are used, so to speak, systematically in applications as time-keeper, requiring a precise time base or when a precise reference frequency is desired, whereas the oscillator circuits used in radio-transmitters generally use LC tank circuits. The limited stability of the latter and the large dispersion of their constitutive components often mandates their frequencies being slaved to a precise reference obtained from a crystal oscillator through a phase lock loop. There is no substantial difference in the circuits associated with these two. types of resonators.
In today's battery operated low power radio systems optimised for low bit-rate communications with duty cycles typically below a few percents, a crystal oscillator has to run in permanence to maintain synchronization and organize a periodic wake-up of the system. The power consumption of this oscillator and its associated division chain is thus a critical parameter since, together with the leakage current of snoozed circuitry (e.g. RAM, μP), it sets the static consumption of the radio system. On the other hand, highly sensitive radio systems require a very accurate and stable time base since an emitter/receiver pair has to synthesize frequencies differing by no more than a few ppm so that a narrowband communication channel can be established. Consequently, such systems require two time bases, a low power one (typically a 32 kHz time base as encountered in wristwatches) that runs permanently and ensures the period wake-up of the radio, and another one relying on a very accurate radio crystal running intermittently but unfortunately at a much higher frequency and thus consuming a lot more power. This arrangement presents however several drawbacks, namely (i) the fact that two external bulky crystals are required, (ii) the fact that the limited precision of the 32 kHz time base implies more frequent wake-up or longer preamble thus increasing the duty cycle of the radio, and (iii) the fact that the accurate time base needs to be powered up for each wake-up thus requiring an extra power consumption and degrading the resonator ageing.
FIG. 1a shows a low consumption oscillator circuit 1 including a quartz resonator typically used in applications as timekeeper or frequency reference. The simplicity and high stability of this oscillator circuit have made it the most popular crystal oscillator structure for low-power time bases found typically in wristwatch applications and for precise frequency reference for radio systems. This oscillator circuit 1, also known as a Pierce oscillator circuit or three-points oscillator circuit, thus includes a branch comprising a series arrangement, starting from a supply potential VDD to a supply potential VSS forming ground, of a current source 2 and a MOS transistor 4 connected via its drain terminal to current source 2 and via its source terminal to potential VSS. A quartz resonator 6 and a resistive element R are connected in parallel between the connection node, indicated by the reference A, of current source 2 and transistor 4, and the connection node, indicated by the reference B, connected to the gate terminal of transistor 4. First and second load capacitive elements C1 and C2 are respectively connected, via one of their electrodes, to connection nodes A and B, the other electrode being connected to supply potential VSS.
A linear analysis of the impedance presented by the circuit of FIG. 1a including the static capacitance of the resonator, CO, leads to a bilinear function of the transconductance gm and thus a circle in the complex plane that allows the determination of the oscillator critical current, the exact frequency of oscillation, its sensitivity to quality factor Q, losses and loading capacitance variations and the maximum start-up current (cf. in particular E. A. Vittoz, M. G. R. Degrauwe and S. Bitz, “High-Performance Crystal Oscillator Circuits: Theory and Application”, JSSC, Vol. 23, No. 3, June 1988, pp. 774-783). To obtain a large circle radius and hence high stability, large loading capacitors are required thus increasing the oscillator power consumption.
If the circuit structure of the 3-points oscillator is duplicated and rendered symmetrical, the cross-coupled pair familiar to differential LC oscillator is obtained. Such a configuration is shown in FIG. 1b. In this example, the circuit uses an LC tank circuit. This oscillator circuit, also globally indicated by the reference numeral 1, includes, placed in parallel between supply potentials VDD and VSS, first and second branches 10, 20 each including a series arrangement of a current source 2, respectively 3, and a transistor 4, respectively 5, connected by its drain terminal to the current source and via its source terminal to potential VSS. The LC tank circuit includes a capacitive element C placed in parallel to the series arrangement of a resistive element R, generally symbolising the arrangement losses, and an inductive element L.
Transistors 4 and 5 are connected in a differential configuration so as to form a crossed pair, i.e. the gate terminal of each transistor is connected to the drain terminal of the other transistor. Connection node A is thus formed of the connection node between current source 2, the drain terminal of transistor 4 and the gate terminal of transistor 5, and connection node B is formed of the connection node between current source 3, the drain terminal of transistor 5 and the gate terminal of transistor 4. The LC tank circuit is thus placed, in a similar manner to the quartz resonator of FIG. 1a, between connection nodes A and B of the oscillator circuit, on the side of the drain terminals of transistors 4 and 5.
The differential structure of FIG. 1b offers substantial advantages for high frequency applications, such as, particularly, reduced sensitivity to the supply and substrate noise, reduced harmonic pair content and limited substrate current injection.
The circuit implementation of FIG. 1b is however not suitable for a resonator exhibiting a high DC impedance such as a crystal resonator since the circuit will merely latch in one of its two stable state, preventing a further oscillation build-up.
Differential oscillator circuit embodiment attempts using an electro-mechanical resonator have been proposed but have not, as yet, led to satisfactory solutions, mainly for reasons of stability. FIG. 1c shows, for example, the prototype of a differential oscillator circuit employing a quartz resonator developed for the first electronic Swiss watch.
This circuit prototype includes two identical branches 10, 20 each including, starting from the supply potential VDD to the supply potential VSS, the series arrangement of a resistive element 8, respectively 9, of an n-MOS transistor 4, respectively 5, and a current source 2, respectively 3. The quartz resonator 6 is connected on the side of the source terminals of transistors 4 and 5 and, in a similar way to the circuit of FIG. 1b, transistors 4 and 5 are connected in a differential configuration, the gate terminal of each transistor being connected to the drain terminal of the other transistor.
As already mentioned, making the differential oscillator circuit of FIG. 1c has not been brought to a successful conclusion for reasons of stability.
If the two sources of the cross-coupled pair are DC separated and capacitively coupled at high frequencies, the structure yields positive feedback only above a given frequency and is thus DC stable. A way to impose a similar biasing current above and below each of the two active transistors has to be found to yield a practical oscillator structure. Several such structures have been identified and are described in the above-mentioned parent U.S. patent application Ser. No. 10/162,714. FIG. 2 shows one of these structures.
The structure depicted in FIG. 2 is, without the crystal resonator and the load capacitance, a well-known relaxation oscillator structure. It has already been proposed in International Application WO 98/48511 in association with an overtone crystal resonator but in a coupled mode where the relaxation is synchronized by the high-Q resonator (cf. also J. R. Westra, C. J. M. Verhoeven and A. H. M. van Roermund, “Resonance-Mode Selection and Crosstalk Elimination Using Resonator-Synchronised Relaxation Oscillators”, Proc. ESSCIRC 98, The Hague, The Netherlands, pp. 88-91).
In addition to the aforementioned oscillator circuits, it will be noted that recent developments in the manufacture of bulk acoustic wave resonators (or BAW resonators), offer new opportunities for applications in the field of telecommunications. Electro-mechanical BAW resonators, and more particularly, thin film BAW resonators have considerable advantages, in particular high working frequencies (of the order of 1 to 10 GHz), a high quality factor, reduced size and the possibility of being integrated directly onto the integrated circuit. By way of complementary information concerning BAW resonators, reference could be made to the document by MM. K. M. Lakin, K. T. McCarron and R. E. Rose, “Solidly Mounted Resonators and Filters”, 1995 IEEE Ultrasonics Symposium, pp. 905-908.
The possibilities offered by the aforementioned BAW resonators require the development of dedicated circuits making use of all the advantages and excellent properties of these resonators.